The writer becalne iliterested in the growth of algae in culture (4, 5, 6, 7, 8, 11, 20, 21, 23, 24, 25) while enlgaoed in attemiipts to isolate a few of the comm1111oni species. In particular the relatioln of these plants to the elenment caleiulmi wvas considered. MOLISCu (15) was the first to report that certain algae call orow normally when calcitmnl is excluded from the miiedia in which they are cultured, while others soo50 die without this elem-lenit. BENECKE (1) also grew a few species in

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... calcium-free culture solutions, anid confirmed the cOlllelUSiOll Of MAOLISCH that calciumii is not essenitial for the g-rowth of some algae. In spite of mianiy iiioculations wi-ith fresh imiaterial of Chiam)iydom)iotias tingens A. BranLn, however, FRANK (9) did niot sueceed iii growi-ing this alga in calcium-deficienit cultures. RICHTER (22) founid that two diatoms, Nitzschia. palea (Kuetz.) W. Sm. anid Naviciula nuiniusclda Grun. also could not grow withonit ealcium. Likewise, MIAERTENS (12) proved that this element was indispenisable for the bluegreens, Oscillatoria bretcis Kuetz. (Oscilla? a bret-is Kuetz.), 0. tenuis Ag., and Nostoe sp., and that strontium could nlot be eompletely substituted for it ill cultures of the last two algae (the first was nlot investigated). WAREN (28) reported that, in general, cell divisions were Inot possible in the desmid Micrasterias rotata (Grev.) Ralfs when there was a deficienyev of calciumii. That a relationship lnight exist between the calcium requlireilments of a plant ancd its systematic position was suggested by PRINGSHEIM (21). Algae whieh could grow\N, without calcium were classed with the lower plants such as the fungi, while those which could not growv w-ithout it were grouped with the hihlier plants. He investigated a large numlilber of algae, some of them producingc no grorwth without calcium and others as Chlorella sp. 1 and 2 developino with or without this element. These latter species continued to develop after several transfers fromii one calcium-deficient solution to another, but notwithstanding the precautions which were taken his results were niot always clear. Amiiong other investigators who have cultivated different strains of Chlorella in solutions with a deficiencey of calcium are HOPKINS anid AWANN (10), TRELEASE and SELSAM (26), PRATT (16, 17), and PRATT and FONG (18, 19). Their works will be referred to later. Materials and methods The orioinal source of the strain of Chlorella selected for this study is uniknloi-own. This alga first appeared oni agar plates which had been inoculated with algoal muaterial from a supply jar in the greenhouLse at Northwestern University. It was cultivated in anld oli a one and one-half per cent. 359 www.plantphysiol.org on August 29, 2017 -Published by Downloaded from